While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF or ArF excimer laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 μm or less.
The chemically amplified resist materials for use in photolithography using light of an excimer laser, especially ArF excimer laser having a wavelength of 193 nm, are, of course, required to have a high transparency to light of that wavelength. In addition, they are required to have an etching resistance sufficient to allow for film thickness reduction, a high sensitivity sufficient to eliminate any extra burden on the expensive optical material, and especially, a high resolution sufficient to form a precise micropattern. To meet these requirements, it is crucial to develop a base resin having a high transparency, rigidity and reactivity. Active efforts have been made to develop such base resins. In addition to these requirements, to minimize a difference in pattern size depending on pattern density (proximity bias) becomes an important task for the current technology.
Known high transparency resins include copolymers of acrylic or methacrylic acid derivatives (see JP-A 4-039665). These copolymers are relatively easy to increase reactivity in that highly reactive monomers can be introduced and acid labile units can be increased as desired. They can also be increased in rigidity by introducing alicyclic groups into acid labile units. Various structures which have been proposed as the alicyclic acid-labile groups to be introduced include alkyl groups with adamantane structure (see JP-A 9-073173), tertiary exo-alkyl groups with bicyclo[2.2.1]heptane framework (see JP-A 12-336121), and mixtures thereof (see JP-A 15-084438). They have reached an acceptable level in satisfying both resolution and etching resistance. However, a further improvement in proximity bias is desired.